The invention relates to electric machinery. In particular, this invention relates to the design of claw pole structures for polyphase electrical machinery.
Electric machinery makes use of a changing magnetic field to produce either an electrical current or a mechanical force. In the case of electric machinery adapted to produce an electrical current, a magnetic field is passed over a wire coil, which induces the desired electrical current in the wire coil. In the case of electrical machinery used to produce a mechanical force, an electric current is passed through a wire coil, which causes the coil to be attracted to (or repelled by) an adjacent magnetic field, thereby yielding the desired force.
The foregoing principles are commonly implemented in rotating electrical machinery. For example, in an electric generator or alternator, a rotating element, or rotor, is passed through the magnetic field produced at intervals by a stator, which has a number of poles arranged around the direction of rotation to provide the magnetic field (which can be generated electromagnetically). An electric motor can have a similar structure; the only difference in principle being that electric current is provided to the rotor coils, rather than being (in the case of a generator or alternator) generated by the rotor coils.
The geometrical design and composition of the stator poles affects the efficiency of operation, as well as the size, shape and weight of the electric machine. A claw-type structure is frequently used for the rotor poles of electrical machines having a single centralized winding or coil. In the case of alternator rotors, the coil is fed by a D.C. current. In other applications, such as in asynchronous motors, stepper motors and brushless permanent magnet motors, the stator coil is fed either by an A.C. current or by impulsions.
The use of laminated materials has constrained the armature geometries of polyphase electrical machines. Typically, the magnetic circuit is constructed by stacking identical laminations one on top of the other, which are electrically isolated from each other to avoid the circulation of eddy currents. These armature geometries are invariant along the axis of rotation. Also, because the magnetic flux circulates in the plan of the laminations, they can be referred to as 2D structures.
The assembly of electrical machines which use a stack of laminations usually requires several additional mechanical parts. For example, it is necessary to add flanges on each opposite side of the lamination stack to support the bearing housings, which perform the function of fixing the rotor to the stator. The assembly of these flanges to the stack is made more difficult by the nature of the laminations and by the presence of the end-windings, which extend out of the slots. These flanges must be distal from the end-windings in order to minimize the flux leakage, if they are made of a magnetic material, or the eddy currents, if they are made of a conductive material. These assemblies usually increase the total axial length of the machine.
Heat dissipation is also a critical problem in the machine structures which use laminated materials, because the heat transfer is much less efficient in the direction perpendicular to the plan of the laminations. Cooling systems, such as an external extruded aluminium yoke equipped with cooling fins, are usually press-fit around the lamination stack to try to improve heat transfer to the ambient atmosphere, but the efficiency of such cooling systems is limited by their poor thermal contact with the laminations. All these problems explain the relatively high number of heterogeneous parts which are necessary in a conventional electrical machine to perform the electromagnetic, mechanical and thermal functions, and which increase its material and assembly cost: windings, laminations, flanges, bearing housing supports, fixing screws and rods, external yoke, aluminium fins, etc.
It is possible to make portions of an electrical machine with an isotropic magnetic material, such as soft magnetic composites made of iron powder. Cooling fins made of the same magnetic material also can be integrated in the magnetic circuit parts. (CA Pat. 2282636 December 1999). It is also known that claw-pole structures present several advantages in low power applications. An example of an electrical machine using a high number of claw-poles are the “canned motors” used in timers or car alternators, which use a claw pole rotor (U.S. Pat. No. 3,271,606 et U.S. Pat. No. 3,714,484). However, this configuration is generally applied to single-phase machines, which use only one coil that is embedded in a magnetic circuit made of two parts equipped with claws. This kind of arrangement is called a “centralized winding”. A centralized winding may be easier to realize than other winding configurations because the total number of coils is generally equal to the number of phases of the motor.
In the case of the inductor of a car alternator, the coil is fed by a DC current. Other applications, such as the stators of asynchronous motors (U.S. Pat. No. 3,383,534), stepper motors (U.S. Pat. No. 5,331,237) and brushless permanent magnet motors (U.S. Pat. No. 5,854,524), illustrate the use of claw-pole structures where the coil is fed either by an AC current or by current pulses.
FIGS. 1A, 1B and 1C show several views of a prior art magnetic circuit component 100; a single phase stator structure. This structure has a yoke 105 (FIG. 1B) that is formed from a magnetic material. The yoke 105 is made from two opposing annuli, 110, 120, with projecting fingers, or claws 112, 114, 116, 122, 124, 126 extending radially inwardly to front along the air gap next to the surface of the rotor (not shown). An annular coil 130 is arranged in the axial direction, concentric with the rotor axis (not shown), surrounded by the two annuli 110, 120 of the magnetic circuit. The plane defined by the coil is perpendicular to the surface of the air gap between the stator and the rotor. The magnetic flux produced by annular coil 130 passes through the air gap to the rotor via the claws 112, 114, 116, 122, 124, 126 on each side of the coil 130.
In the case of an AC claw-pole armature, it is preferable to realize the core with a composite magnetic material to minimize the eddy current losses (U.S. Pat. Nos. 3,383,534 and 5,331,237). One can also use an assembly of magnetic sheets, or laminations, and other parts made from iron-powder materials, produced by powder metallurgy methods (U.S. Pat. Nos. 6,320,294, 6,201,324).
The implementation of a polyphase structure with a claw-pole armature is usually more difficult. It is necessary to stack several single-phase structures placed on the stator or the rotor and separate them by air gaps to avoid magnetic short circuits and performance degradations. However, in the case of small-power motors, such as stepper motors, one can often tolerate this kind of degradation of performance by directly juxtaposing several single-phase structures without adding air gaps between the structures (U.S. Pat. Nos. 6,259,176 6,031,304). U.S. Pat. No. 5,854,526 illustrates a three-phase, claw-type structure using a different arrangement of the coils. Three coils are placed in the same plan, with their axes parallel to the surface of the air gap between the stator and the rotor. As in the case of the preceding structures, the plan defined by the coils is perpendicular to the air gap surface. However, these structures do not solve the problems of magnetic short circuits and should only be used in applications with a very small power.